This invention relates to a process for reducing the vanadium content of an aqueous stream. In particular, the invention relates to passing a sodium chromate or sodium bichromate liquor containing dissolved vanadium over an insoluble trivalent chromium compound.
The principal uses for sodium bichromate are in leather tanning, metal finishing, chrome metal manufacturing, and chromate pigments manufacturing. In leather tanning, the sodium bichromate in the liquor is converted to basic chrome sulfate, which is then used in tanning the leather. Even small amounts of vanadium (e.g., less than 1000 ppm) in the bichromate liquor can result in discoloration of the leather. Sodium bichromate is used to manufacture Cr.sub.2 O.sub.3 which in turn is converted into chrome metal. Chrome metal is used for making super alloys, which are high temperature, high strength alloys of chromium used in the turbine blades of jet engines. The properties of these alloys are very sensitive to the presence of vanadium and other metal contaminants. Vanadium concentrations in excess of about 100 ppm are not desirable. Vanadium in sodium bichromate at concentrations of &lt;100 ppm may also affect the properties of chromate based pigments.
Sodium bichromate liquor is now being made commercially by roasting chromite ore, leaching sodium chromate from the ore, and filtering off the solids. The presence of lime in the ore results in the precipitation of calcium vanadate, which is removed with the solids in the filtration. This reduces the vanadium concentration of the liquor to about 120 ppm. The sodium chromate liquor is then acidified with sulfuric acid and is partially evaporated to precipitate sodium sulfate and form a sodium bichromate liquor. Some of the sodium bichromate liquor is sold or used to make chromic acid and the remainder is cooled, which results in the precipitation of sodium bichromate crystals. The remaining liquor is then recycled to the evaporation stage.
Since little of the vanadium that is in the liquor is entrained with the sodium bichromate crystals, as the sodium bichromate liquor is recycled and new vanadium-containing liquor is processed, the vanadium concentration in the liquor tends to increase. As the vanadium concentration in the sodium bichromate liquor builds up, the amount of vanadium in the sodium bichromate crystals increases. If the production and sale of sodium bichromate liquor is large compared to that of the crystals, then the liquor sales provide an adequate purge for vanadium at levels below customer specifications. However, if the production of liquor is reduced, or the production of crystals is increased, then the concentration of vanadium in the liquor may become unacceptably high without a secondary removal method. Thus, under certain market conditions, a method is needed for removing vanadium either from the recycled sodium bichromate liquor or in the sodium chromate solution which ultimately feeds the bichromate process. Until now, little technology was available for producing a concentrated sodium bichromate liquor containing less than 100 ppm of vanadium. One method was to crystallize the sodium bichromate and redissolve the crystals, a procedure that is too costly to be of much commercial value. Even ion exchange resins are ineffective in removing vanadium from concentrated bichromate liquors because the liquor attacks the ion exchange resin.